The invention relates to a flow control subsystem for a fuel cell system.
A fuel cell is an electrochemical device that converts chemical energy produced by a reaction directly into electrical energy. For example, one type of fuel cell includes a proton exchange membrane (PEM), often called a polymer electrolyte membrane, that permits only protons to pass between an anode and a cathode of the fuel cell. At the anode, diatomic hydrogen (a fuel) is reacted to produce hydrogen protons that pass through the PEM. The electrons produced by this reaction travel through circuitry that is external to the fuel cell to form an electrical current. At the cathode, oxygen is reduced and reacts with the hydrogen protons to form water. The anodic and cathodic reactions are described by the following equations:
H2xe2x86x922H++2exe2x88x92 at the anode of the cell, and
O2+4H++4exe2x88x92xe2x86x922H2O at the cathode of the cell.
A typical fuel cell has a terminal voltage near one volt DC. For purposes of producing much larger voltages, several fuel cells may be assembled together to form an arrangement called a fuel cell stack, an arrangement in which the fuel cells are electrically coupled together in series to form a larger DC voltage (a voltage near 100 volts DC, for example) and to provide more power.
The fuel cell stack may include flow plates (graphite composite or metal plates, as examples) that are stacked one on top of the other, and each plate may be associated with more than one fuel cell of the stack. The plates may include various surface flow channels and orifices to, as examples, route the reactants and products through the fuel cell stack. Several PEMs (each one being associated with a particular fuel cell) may be dispersed throughout the stack between the anodes and cathodes of the different fuel cells.
The fuel cell stack may be part of a fuel cell stack system that supplies electrical power to an electrical load. For example, for a residential fuel cell system, the electrical load may be established by the various power consuming devices of a house. To furnish AC power to the house, the fuel cell system typically converts the DC voltage that is provided by the fuel cell stack into AC voltages. The fuel cell system may include a fuel processor to convert a hydrocarbon (natural gas or propane, as examples) into a reformate flow that furnishes the hydrogen to the fuel cell stack. The fuel cell system may also include an air blower that produces an air flow that furnishes the oxygen to the fuel cell stack.
For various reasons, it is desirable in a fuel cell system to be able to bypass reactant flows from the fuel cell stack. For example, upon start-up of a fuel processor, the initial reformate stream may contain high levels of carbon dioxide that would damage the electrode catalysts of the fuel cell stack. On start-up, it may thus be desirable to burn any off-specification fuel in a flare. Bypassing fuel to a flare on start-up may also provide system warm-up capabilities for outdoor systems in cold climates. Other operating modes, such as routine and emergency shut down scenarios, are also provided through fuel bypass capabilities. It will be appreciated that the performance, reliability and efficiency of a fuel cell system is increased by improving the performance reliability and efficiency of the reactant flow control subsystem of the fuel cell system. Likewise, as fuel cell technology is transitioned into consumer products, it is also desired to have such a flow control subsystem that is inexpensive and easy to manufacture.
There is a continuing need for an arrangement in a fuel cell system that efficiently and dependably addresses one or more of the issues stated above.
An automated reactant flow control subsystem is provided for a fuel cell system. The subsystem is achieved with a minimum of parts for decreased cost and increased reliability. The subsystem includes a fail-safe solenoid-actuated three-way valve in the fuel line that achieves very low pressure drop and very low parasitic load requirements. The subsystem also includes a fuel bypass system such as a flare, and a controller to automatically interlock the fuel and oxidant streams of the fuel cell.
In general, in one embodiment, the reactant flow control system includes a three-way valve adapted to selectively switch a fuel stream between a bypass path and a fuel cell stack path. The valve is connected to a supply line, a bypass line, and a stack line, and has a bypass line seating orifice and a stack line seating orifice. A plunger in the valve directly abuts the stack seating orifice when the valve is in a bypass position, and directly abuts the bypass seating orifice when the valve is in a operating position. The bypass position is used to divert the fuel stream away from the fuel cell, for example, to flare off-specification fuel on start up or shutdown of the fuel cell system. The operating position is used to supply the fuel stream to the fuel cell during normal operation.
An important feature of the design is that the valve is configured to achieve very low pressure drop when in the operating position. For example, the stack line seating orifice is sized to have a cross-sectional area that is larger than the cross-sectional area of the stack line (for example 120% or larger) such that when the valve is in the operating position, the pressure drop across the valve is less than 5 inches water column (IWC) at a fuel stream flow of 20 cubic feet per minute (CFM). In some embodiments, the stack line seating orifice has about the same cross-sectional area as the stack line, and the pressure drop may be as low as 0.5 IWC at 20 CFM of fuel flow through the valve. Another feature of the design is that the plunger within the valve directly abuts the stack line seating orifice, and the orifice leads directly to the stack line. In this manner, when the valve is in the operating position, the fuel flow through the valve has a more direct path and lower pressure drop than in conventional 3-way valve designs, such as those typical in hydraulic systems where the flow path through such valves is often circuitous and restricted. The valve housing and plunger shape, which generally define the flow path through the valve, are also configured to provide a smooth and direct flow path through the valve to promote laminar flow through the valve.
The low pressure drop aspect of the above-described design features makes such a system advantageous for a fuel cell system that is operated at close to atmospheric pressure (for example, less than one atmosphere), since less energy is required to push reactants through the system.
Another important feature of the design is that it is configured to achieve failsafe operation with minimum power requirements. The failsafe operation refers to the fact that the plunger in the valve is biased to the bypass position. Thus, as an example, if the overall system were to lose power, the flow control system could bypass the fuel stream to a flare system. The bypass position of the valve can thus be referred to as the non-energized position, and the operating position can thus be referred to as the energized position.
In some embodiments, the valve is solenoid actuated by an electromagnetic coil surrounding the stem of a spring-loaded plunger. When a sufficient power is supplied to the coil (for example 10 Watts), the resulting electromagnetic force compresses the plunger spring, placing the valve in the operating position while the power is supplied. In other embodiments, the power requirements of such operation are minimized by utilizing a second coil around the plunger stem. The second coil is used to hold the plunger in the operating position since this requires less power (for example 5 Watts) than the power needed to actuate the plunger. The lower power requirements of the second coil thus replace the higher power requirements of the first coil during normal operation of the system. The power required to maintain operation of the fuel cell system may be referred to as the parasitic load. The two coil approach provides increased system efficiency by reducing the parasitic load on the system.
Finally, in some embodiments, the flow control subsystem is associated only with the fuel lines of the system. Whereas a need may exist to be able to bypass off-specification fuel (for example, fuel that is high in carbon monoxide, which would damage the fuel cell electrode catalysts), a similar need may not exist to bypass the oxidant gas. For example, in a start-up or shut-down operating mode, fuel might be bypassed away from the stack to a flare system, and the oxidant stream of the system might continue flowing through the stack on its way to the flare. In other embodiments, the flow control subsystem may be associated with both the fuel lines and the oxidant lines to bypass all reactant flows from the stack when desired. For example, in a PEM system where membrane dry-out is a concern, it may be desirable to bypass sub-saturated oxidant flow from the stack on start-up.
Advantages and other features of the invention will become apparent from the following description, from the drawing, and from the claims.